Multi-layer ceramic (MLC) structures are used in the production of electronic substrates and devices. The MLCs can have various layering configurations. For example, a MLC circuit substrate may comprise patterned metal layers which act as electrical conductors sandwiched in between ceramic layers which act as a dielectric medium. For the purposes of interlayer interconnections, most of the ceramic layers have tiny holes or via holes. Prior to lamination, the via holes are filled with an electrically conductive paste, such as, a metallic paste, and sintered to form vias which provide the electrical connection between the layers. In addition, the MLC substrates may have termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, to name a few.
Generally, ceramic structures are formed from ceramic green sheets which are prepared from a slurry of ceramic particulate, thermoplastic polymer binders, plasticizers, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheets. After punching, metal paste screening, stacking and laminating, the green sheets are fired or sintered at temperatures sufficient to burn-off or remove the unwanted polymeric binder resin and sinter the ceramic particulate together into a densified ceramic substrate. The present invention is directed to the screening, stacking and lamination steps of this process.
In the MLC packaging industry it is very common to use green sheets of various thicknesses. The thicknesses can typically vary from 6 mils to 30 mils and in general the art of punching and metallizing these layers are well known. Green sheet thicknesses below 6 mils, in general, are very scarcely used. This is due to a variety of reasons, such as, for example, handling, screening and stacking of green sheets thinner than 6 mils pose tremendous challenges. In fact the use of one to two mils thick ceramic green sheets, which are punched and screened, using traditional MLC technology does not exist.
Also, in the MLC packaging industry it is very common to use capacitor layers. The capacitance necessary in a package depends on the design and such capacitance is obtained by choosing proper dielectric layer thickness and metal area within a layer. The industry is always striving for higher capacitance and since the metal area is maxing out for a given substrate size it is necessary to use thinner dielectric layers between electrodes to obtain the required capacitance. For example, as a rule of thumb one could double the capacitance for a given dielectric system and electrode metal area by decreasing the dielectric layer thickness by half. Additionally the number of layers needed for capacitance in a package as well has been reduced by about 50 percent. The reduction in the number of layers is desirable, as it reduces the cost and the process of making the substrate.
U.S. Pat. No. 5,254,191 and No. 5,474,741 (Mikeska) teaches the use of flexible constraining layers to reduce X-Y shrinkage during firing of green ceramic bodies. But these flexible constraining layers, among other things, do not act as an adhesion barrier during lamination.
However, the present invention forms laminated multi-density, multi-layer ceramic structures using at least one very thin green sheet and/or at least one green sheet with very dense electrically conductive patterns on top of at least one thicker green sheet. An adhesion barrier that is useful during the lamination process is also utilized to build these multi-density, multi-layer ceramic laminates.